1. Field of the Invention
The present invention relates to a failure diagnosis apparatus for evaporative purge system which diagnoses failures in an evaporative purge system extending from a fuel tank to a purge passage by way of a canister.
2. Related Background Art
Conventional failure diagnosis apparatus for evaporative purge system is disclosed in Japanese Patent Application Laid-Open No. 5-180099. FIG. 20 shows such an apparatus for evaporative purge system.
First, in the diagnosis operation in this apparatus, a second control valve 101 serving as an air release valve is closed so as to shut an air opening of a canister 102, while a first control valve 103 is opened. Consequently, a negative pressure of an intake passage 104 is introduced into an evaporative purge system extending from a purge passage 105 to a fuel tank 107 by way of the canister 102 and an vapor passage 106. Thereafter, under a predetermined condition, the first control valve 103 is closed so as to temporarily cut the purge and, during this period, the pressure behavior within the system extending from the first control valve 103 to the fuel tank 107 is measured. After purge-cutting, the pressure behavior within this system, measured as the canister internal pressure, gradually increases toward the atmospheric pressure. When leakage occurs within the system due to perforation therein, for example, as shown in FIG. 21, pressure change .DELTA.P.sub.1 per predetermined time t is greater than pressure change .DELTA.P.sub.0 in the case where no leakage occurs. By utilizing this phenomenon, the apparatus measures the pressure behavior after the purge-cutting, thereby diagnosing the failure within the subject system.